Epsilon caprolactam, usually referred to simply as "caprolactam", is a large volume commodity chemical used as a monomer in the production of the commercially important Nylon-6. Although routes to the precursor cyclohexanone oxime vary, all commercial caprolactam production makes use of a Beckmann rearrangement of the oxime. The commercial reaction is carried out in a batch operation in oleum (H.sub.2 SO.sub.4 SO.sub.3) solution. The recovery step in this technology employs an ammonium hydroxide neutralization of the resulting caprolactam-oleum solution, a process generating two moles of by-product ammonium sulfate per mole of product. The sulfate has some value as a low grade fertilizer, but its recovery and/or disposal can add substantial cost to Nylon-6 production. Attempts have been made to circumvent the use of oleum and carry out the reaction in the gas phase, thereby eliminating the undesirable by-product.
A number of patents and publications have appeared which describe such heterogeneous gas phase conversions. Examples of these include U.S. Pat. No. 3,503,958 to P. S. Landis, which describes and claims such conversion using a zeolite such as hydrogen Y; U.S. Pat. No. 3,016,375 which uses as catalyst polyphosphoric acid; and U.S. Pat. No. 4,359,421 to Bell et al. which uses as catalyst a zeolite having a silica to alumina ratio of at least 12 and a Constraint Index of 1 to 12.
U.S. Pat. No. 4,582,815 to Bowes describes and claims a method for preparing binder-free and silica-bonded extrudates of zeolites, including ZSM-5.
U.S. Pat. No. 4,697,010 to McMahon discloses a process for the catalyzed conversion of cyclohexanone oxime to caprolactam over a catalyst composition comprising the hydrogen form of crystals having the structure of ZSM-5 made in the presence of a boron source.
U.S. Pat. No. 4,709,024 to Sato et al. discloses a process for the production of epsilon caprolactam using a crystalline aluminosilicate catalyst having specific Si/Al atomic ratio and a specific acid amount of external surface.
U.S. Pat. No. 4,927,924 to Bell et al. discloses synthesis of caprolactam using a catalyst subjected to steam treatment to reduce its Alpha Value.
The desired rearrangement of cyclohexanone oxime (I) to caprolactam (II) is believed to occur via a protonated intermediate (not shown), according to Equation A. ##STR1##
In addition, however, two major side reactions occur, one with the formation of 5-cyanopentene (III) and water (Equation B), and the other forming cyclohexanone oxime (IV) (Equation C). ##STR2##
In addition to (III) and (IV) other by-products of unknown structure also are formed.
It is an object of the present invention to provide a heterogenous process which is highly selective for the desired caprolactam product. It is expected that an increase in selectivity of the zeolite catalyst is obtained when cyclohexanone oxime is converted to epsilon caprolactam using a zeolite catalyst having reduced surface acidity to minimize non-selective reactions on the surface acid sites of the zeolite catalysts. It is a further object of the present invention to improve process performance of the zeolite catalyst used in caprolactam synthesis, i.e., better (slower) aging.